High speed extrusion

ABSTRACT

Extrusion rheology of thermoplastic polymer as manifested by surface smoothness is improved by incorporating foam cell nucleating agents into the polymer and extruding the polymer under laminar flow to form unfoamed extrudate.

This is a division of application Ser. No. 08/632,376, filed Apr. 10,1996, which is now U.S. Pat. No. 8,688,457.

FIELD OF THE INVENTION

This invention relates to the use of additives to increase extrusionrate of thermoplastic polymer.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 2,991,508 discloses the extrusion of smooth-surfacedarticles such as film and wire coatings of thermoplastic polymer. As theextrusion rate increases to reach the critical extrusion rate, thesurface of the extrudate becomes rough. Upon further increase inextrusion rate, the super shear rate is reached, at which the extrudatesurface becomes smooth again. As the extrusion rate is furtherincreased, the extrudate surface becomes rough again, representing themaximum extrusion (shear) rate for the polymer. Usually, the surfaceroughening just above the critical shear rate has the appearance ofsharkskin, i.e. a satin appearance, with the roughness of the surfacesimilar to the texture of very fine sandpaper, barely perceptible to thetouch. The severity of the roughness may increase to the point where thesurface roughness is clearly visible as surface irregularities and thesecan be felt by touch. This surface roughness is commonly referred to asgross melt fracture. The surface roughening occuring as the extrusionrate is increased above the super shear rate is gross melt fracture.Thus, it is gross melt fracture that usually forms the limitation on theshear of the polymer and thus its maximum extrusion rate. Some polymersdo not exhibit the super shear phenomonon, i.e. the the extrudatesurface transforms from being smooth to sharkskin to gross meltfracture, while other polymers transform from smooth surface to grossmelt fracture. For many applications, just the sharkskin appearance isobjectionable and the extrusion rate at which this occurs forms themaximum extrusion rate.

U.S. Pat. No. 3,125,547 discloses the use of certain fluoropolymers asprocessing aids in the extrusion of hydrocarbon polymers. This patentwas followed by improvements in the fluoropolymer used as the processingaid and combinations of fluoropolymer with certain additives, such aspolyethylene glycol and or polar-side-group adjuvants. Generally, theprocessing aids are effective in postponing the occurence of sharkskinas extrusion rate is increased, but have little effect on the extrusionrate at which gross melt fracture occurs.

Improvement is still needed in the maximum extrusion rate attainable forthermoplastic polymer which is extrusion rate limited by extrudatesurface roughness, whether sharkskin or especially, gross melt fracture.

SUMMARY OF THE INVENTION

The present invention involves the discovery that foam cell nucleatingagents when added to the thermoplastic polymer act as very effectiveprocessing aids to enable the maximum extrusion rate to be increasedbefore the extrudate surface changes from smooth to rough. Foam cellnucleating agents are normally used to nucleate the formation of voidsin polymer extrudate, so that a foamed extrudate containing small cellsis formed by virtue of the presence of blowing agent in the molten resinat the time of extrusion. The foamed extrudate will usually have a voidcontent of at least 20%. U.S. Pat. No. 4,764,538 discloses that boronnitride has been the nucleant of choice for foaming fluoropolymers andthat the addition of certain amounts of inorganic salts gives enhancedfoam nucleation.

In the melt extrusion process of the present invention, foam cellnucleating agent is present in the polymer, but without blowing agentbeing present at the time of extrusion, so that the extruded polymer isunfoamed.

Thus, the process of the present invention comprises melt extrudingunfoamed extrudate of thermoplastic polymer at a shear rate which is atleast 1.2 times the shear rate at which said extrudate would otherwisebecome rough-surfaced, said polymer having incorporated therein aneffective amount of foam cell nucleating agent to enable extrudate tohave a smooth surface.

In one embodiment of this process, the shear rate is at least 1.2 timesthe shear rate at which the extrudate normally exhibits gross meltfracture. In this embodiment, the extrudate is either smooth surfaced ormay have sharkskin in applications where such appearance is permitted.Preferably, the shear rate is at least 1.5 times the shear rate at whichthe extrudate would otherwise exhibit gross melt fracture and theextrudate is smooth surfaced.

The reference to the shear rate at which the extrudate exhibits surfaceroughness, should be understood to mean the shear rate at which theonset of roughness occurs when the foam cell nucleating agent is notpresent.

Another requirement of the process of the present invention is that theextruding of the thermoplastic polmer to form the unfoamed extrudate iscarried out so that the molten polymer is under laminar flow conditionas it forms the extrudate, i.e. as it exits the extrusion die. Theextrusion carried out under laminar flow condition in combination withthe presence of the effective amount of foam cell nucleating agent thatprovides the improved extrudibility of the present invention.

The significance of the laminar flow condition is seen from the factthat determination of the super extrusion shear rate in the '508 patentwas carried out using a laboratory rheometer, which is the typicalequipment used for testing extrudability, prior to using a commercialextruder. The improved extrudability of the present invention was notobtained using the rheometer on fluoropolymer composition containing thefoam cell nucleating agent, but was obtained when using a commercialextruder having a crosshead for wire coating with the polymer extrudateof the same composition. Thus, it was surprising that the shear rate forthe polymer containing the nucleating agent could be increased beyondwhat was predicted to be the maximum shear from rheometer experiments.It was determined that the equipment geometry was responsible for theimproved results. The extruder crosshead provided laminar flow of themolten resin as it formed the extrudate. The rheometer had a relativelylarge die inlet angle, i.e. 90 degrees, which created turbulence in themolten polymer just prior to extrusion, and this turbulence manifesteditself as gross melt fracture present in the extrudate.

The present invention also provides several new resin compositions whichcan be in either the pre-extruded state as extrudible composition orpost extrusion as the extrudate in such forms as wire insulation,jacketing, tubing, rod and film. In one embodiment of the composition,the thermoplastic polymer is fluoropolymer and the foam cell nucleatingagent is boron nitride. U.S. Pat. No. 4,764,538 discloses the amount ofboron nitride to be used for nucleating foam cells in fluoropolymer isat least 0.5 wt % when used by itself and at least 0.05 wt % when usedwith 25 to 1000 ppm of inorganic salt which is thermally stable at theextrusion temperature. In commercial foaming practice, the amount ofboron nitride used has been about 0.25 wt % and the amount of inorganicsalt used has been about 100 ppm. In this compositional embodiment ofthe present invention, the amount of boron nitride effective to improvethe extrusion performance of the fluoropolymer to make unfoamedextrudate can be less than 0.02 wt %.

In another embodiment of the composition, the thermoplastic polymer ispolyolefin and the foam cell nucleating agent is boron nitride alsopresent in an amount which can be less than 0.02 wt % to improveextrusion performance to make unfoamed extrudate.

DESCRIPTION OF THE DRAWING

The FIGURE shows a side view in cross-section of an extrusion dieadapted to provide laminar flow of molten polymer forming unfoamedextrudate.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In the FIGURE, an extrusion die 2 is shown having an internal diepassage 4 which terminates in an orifice 8 through which moltenthermoplastic polymer 10 is extruded. The molten polymer is supplied byan extruder (not shown) which forces the molten polymer into die 2through annular inlet port 11 entering the die from its side. The die 2is shown in indeterminate length, indicating that the inlet port 11 isspaced considerably from the orifice 8. The die 2 has a conical interiorsurface 12 which tapers inwardly to form a smooth transistion with theinlet to the die passage 4. A wire guide 16 is positioned within theconical interior surface 12 and has a central passage 18 through whichwire 20 passes in the direction indicated, a conical exterior surface22, and a cylindrical extension 24 which forms a smooth transistion withsurface 22 extending the length of the die passage 4 and terminating atthe die orifice 8. The die orifice 8, die passage 4, conical interiorsurface 12, and wire guide 16 are concentrically positioned andsymmetrical about the axis 6. The included angle of the conical interiorsurface 12 is indicated as 14 in the FIGURE, and can be referred to asthe die inlet angle. The included angle of the conical surface 22 of thewire guide 16 is indicated as 28 in the FIGURE. Collectively, theseangles may be referred to as the conical angles.

The molten polymer enters the die 2 via port 11 and is forced around thewire guide 16 towards the die orifice 8. The wire guide serves as amandrel for the molten polymer, giving the extrudate 10 a tubular shape.The die passage 4 forms the exterior surface of the tubular shape andthe exterior surface of the cylindrical extension 24 forms the interiorsurface of the tubular shape. The greater speed of the wire as comparedto the polymer extrusion rate causes the polymer coming into contactwith the wire at a point remote from the die orifice 8 to draw down to athinner cross-section, forming a thin polymer coating 26 on the wire,which will serve as electrical insulation. This is a melt draw-downextrusion process, and the draw down ratio (ratio of die orifice area tocross-sectional area of the polymer insulation) of the polymer willgenerally be at least 5:1, although the process of the present inventionis applicable to pressure extrusion in which there is no draw down.

In the embodiment shown in the FIGURE, laminar flow of the moltenpolymer is achieved by the time the molten polymer enters the die inlet,which is the transistion between the conical interior surface 12 of thedie 2 and the die passage 4. The molten polymer thus flows laminarlyalong the die passage and out the die orifice 8 enabling the improvedextrudibility of the present invention to be obtained, i.e. when thethermoplastic polymer contains the foam cell nucleating agent. Grossmelt fracture is accompanied by turbulence within the molten polymer atthe die inlet, which is indicative of a region of very high shearstress. Laminar flow of the molten polymer in accordance with thepresent invention is a consequence of low shear stress being present inthis region. The turbulence within the molten polymer can be seen whenthe extruder material of construction in the region of the die inlet isglass so that the movement of the molten resin within this region can beseen. Such turbulence includes the molten resin even reversing itsdirection of movement through the extruder at the die inlet, in thenature of eddy currents. Since extruders and extrusion dies are notnormally made of glass as the material of construction, the presence ofthis turbulence cannot normally be seen, but is deduced as being presentwhen the extrusion produces a rough-surface extrudate. Similarly, theabsence of turbulence, i.e., the presence of laminar flow, is deducedfrom the smooth surface of the extrudate. This laminar flow may take theform of the molten resin moving into and through the die at a fasterrate in the center of the extrudate then the resin moves along the diepassage, i.e. the molten resin may tend to adhere to the wall of the diepassage. Laminar flow may also take the form of "plug flow", wherein themolten resin slips along the wall of the die passage rather than adhereto it, so the resin moves as a plug along the die passage.

Laminar flow in the molten polymer entering the die inlet is achieved bysubstantial conformation of the coaxially interfitted conical surfaces12 and 22, i.e. substantial conformation of the die inlet angle 14 andthe included angle 28 of the conical surface 22 of the wire guide.Preferably the included angle of the conical surface 22 or mandrel iswithin 20 degrees of the die inlet angle, more preferably within 10degrees thereof. In the embodiment shown in the FIGURE, the die inletangle is about 60 degrees. Preferably, the die inlet angle is 30 to 70degrees, and most preferably, the conical angles (included angle of theconical surface 22 and die inlet angle) are substantially the same, withthe conical surfaces 12 and 22 being substantially parallel to oneanother, whereby the annulus between the conical surfaces is of uniformwidth along the pathway for the molten polymers. The result of thisconformation of the conical surfaces is that the molten polymer enteringthe die via port 11 envelops the mandrel and flows along the conicallyconverging annular path defined by surfaces 12 and 22, to enter the dieinlet essentially without turbulence, for extrusion through the dieorifice without the occurrence of unfoamed extrudable surfaceroughening.

While the presence of laminar flow is established by equipment geometrywhich streamlines the flow of molten polymer into the die inlet,achievement of the laminar flow is judged by the extrusion result, i.e.the nucleating agent in the thermoplastic polymer enables the extrusionrate to be increased beyond the maximum at which surface roughening ofthe extrudate otherwise occurs, without obtaining the surfaceroughening.

While laminar flow of the polymer is important to achieving asmooth-surface extrudate, it is also important that the foam cellnucleating agent be present in an effective amount. It is thecombination of laminar flow of the molten polymer within the extrusiondie as the polymer enters the die passage, with the presence of the foamcell nucleating agent that enables the shear rate for smooth-surfaceunfoamed extrudate to be increased beyond the maximum shear rateattainable for smooth surface extrudate when only laminar flow ispresent.

The extrusion die 2 shown in the FIGURE is used for wire coating and issituated in a cross-head positioned at the outlet end of a wire-coatingextruder (not shown), wherein the molten polymer exits the extruder viaport 11 into the die arranged at a right angle to the path of the moltenpolymer exiting the extruder, so that a wire may be passed through thedie for eventual contact by the polymer as shown in the FIGURE. The wiremay be omitted and the extrudate will be in the form of tubing.

If the wire guide 16 were omitted altogether, in which case the die 2could be positioned in line with the longitudinal axis of the extruderinstead of perpendicular as in the case of the crosshead arrangement,the molten polymer would have turbulence as it enters the die passage 4,and the improved extrusion performance will not be obtained. Such anextruder/die arrangement would operate similar to a rheometer asdescribed above. The present invention, however, includes that discoveryof foam cell nucleating agents which enable such apparatus to yieldimproved extrusion performance, i.e. the effectiveness of these agentsis such that a wider range of die configuration and geometry can be usedto provide the laminar flow necessary to obtain the improved results.

Achievement of the laminar flow condition is normally establishedupstream of the die passage 4, otherwise the length of the die passagewould have to be inordinately long. As indicated above, the laminar flowcondition is obtained by providing a pathway for the molten polymerwhich feeds the polymer to the die inlet. Preferably, the combination ofthe laminar flow condition achieved by extrusion equipment geometry andthe presence of an effective amount of the foam cell nucleating agentare such that the shear rate can be increased by at least 2 timesgreater than the maximum shear rate at which surface roughness wouldotherwise occur, but without surface roughening of the extrudateoccuring, and more preferably, this effect is achieved with respect tothe occurence of gross melt fracture. Increases in shear of at least 10×are possible in operations under the present invention.

The extrusion die shown in the FIGURE is one way of forcing moltenpolymer through an orifice. The orifice need not be circular but canhave other annular shapes. The extrusion process of the presentinvention is also applicable to the operation of other apparatus whichforces molten polymer through an orifice, e.g. injection molding, blowmolding, extrusion of film, including sheet. One skilled in the art,will be able to adapt the various apparatus possibilities to producelaminar flow of the molten polymer as it exits the extrusion orifice,regardless of its shape.

The surfaces of the unfoamed extrudate need to be smooth on the interioras well as the exterior, even in the case of wire coating, so that theresulting insulation on the wire is in intimate contact with the wire.Generally, if the exterior surface is smooth, the interior surface willalso be smooth, so that it is usually only necessary to monitor theexterior surface of the extrudate for smoothness. In the initialequipment setup, however, it may be desirable to examine the interiorsurface to confirm its smoothness under shear rate at which the exteriorsurface is smooth.

Boron nitride is a preferred foam cell nucleating agent because of itsapplicability to a broad range of thermoplastic polymers, e.g., bothfluoropolymer and polyolefin, and because it seems to provide thegreatest improvement in extrudability (extrusion rate) above the rate atwhich the extrudate surface otherwise becomes rough. Use of boronnitride is especially effective in enabling the increase in shear rate,and thus production rate, to produce smooth-surface unfoamed extrudateof at least 10 times greater than the shear rate at which the extrudatesurface otherwise becomes rough. The preferred amount of boron nitrideeffective to provide improved extrusion performance to obtain unfoamedextrudate can be as little as from 0.001 to 0.015 wt %, more preferablyfrom 0.005 to 0.01 wt %. Thermoplastic polymers containing thesequantities of boron nitride are additional embodiments of the presentinvention, with fluoropolymers and polyolefins, being the preferredpolymers. Thermoplastic polymers containing the low amounts of BN, insteady state extrusion, may be advantageously preceded by a short run ofhigher BN content of the same polymer to condition the extruder.

Thermally stable inorganic salt may also be used in combination with theboron nitride. Generally the amount of inorganic salt used will be from0.001 to 0.05 wt %, preferably from 0.002 to 0.04 wt %. Examples ofinorganic salts include calcium tetraborate, sodium tetraborate,potassium tetraborate, calcium carbonate, and zinc tetraborate.

The present invention has also found that many other foam cellnucleating agents are operable to enable the extrusion shear rate to beincreased without causing surface roughness of the unfoamed extrudate.At least one foam cell nucleating agent is used and, often a combinationof foam cell nucleating agents are used. Such foam cell nucleatingagents can be organic or inorganic and all have thermal stability underthe conditions of extrusion, i.e. they do not decompose to cause bubbleformation. They are also solid under the extrusion conditions, exceptthat certain foam cell nucleating agents may at least partially dissolvein (disappear into) the molten polymer as will be hereinafter described.

Examples of organic foam cell nucleating agents include low molecularweight polytetrafluoroethylene, often called PTFE micropowder, the lowmolecular weight being characterized by a melt viscosity of 1×10³ to1×10⁵ Pa.s at 372° C. This nucleating agent is more effective inpolyolefins as compared to fluoropolymers.

Additional examples of nucleating agents include the fluorinatedsulfonic and phosphonic acids and salts disclosed in U.S. Pat. No.5,023,279, such as Telomer B sulfonic acid having the formula F(CF₂)_(n)CH₂ CH₂ SO₃ H, wherein n is an integer of 6 to 12, wherein theparticular Telomer® B is identified by the predominant value of theinteger "n", e.g. BaS-10 is the barium salt of the sulfonic acid whereinn=10 as the predominant chain length present. Additional salts includeBaS-8, ZrS-10, CrS-10, FeS-10, CeS-10, and CaS-10. For lower meltingthermoplastic polymers, e.g. polyolefin, hydrocarbon salts of these longchain sulfonic or phosphonic acids can be used, such as BaS-3H andKS-1(H), i.e. barium propane sulfonate and potassium methane sulfonate,respectively. The eight-carbon perfluorinated sulfonic acid available asFluororad® FC-95, can also be used These acids and salts are believed toat least partially dissolve in the molten fluoropolymer.

Examples of inorganic foam cell nucleating agents include boron nitrideas mentioned above, calcium tetraborate, talc, and metal oxides such asMgO, Al₂ O₃, and SiO₂. The amount of foam cell nucleating agent usedwill depend on the particular nucleating agent used, the host polymer,and the increase in shear (production) rate desired above the maximumshear rate obtainable before surface roughening, such as gross meltfracture, is evident at the surface of the extrudate, when thenucleating agent is not present. Generally, 0.001 to 5 wt % foamnucleating agent can be used. Linear polyethylene seems to require onlyabout 0.05 to 0.75 wt % of nucleating agent, except that boron nitridewith and without thermally stable inorganic salt can be used in muchsmaller amounts as described above. The same is true for fluoropolymers.All %s disclosed herein are weight percent and are based on the totalweight of the polymer plus nucleating agent. The foam cell nucleatingagent may be a combination of inorganic agents or organic agents or acombination of inorganic and organic foam cell nucleating agents.

For fluoropolymers, the amount of nucleating agent will generally befrom 0.001 to 0.5 wt %. The nucleating agent can be added to the polymerin the extruder or can be dry-mixed therewith prior to extrusion, withthe goal being to obtain a uniform distribution of the nucleating agentwithin the molten polymer at least just prior to extrusion. Thenucleating agent can be added to the polymer undiluted or the nucleatingagent may be in the form of a concentrate of the nucleating agent inpolymer (resin) which is the same as or is compatible with the polymerto be eventually extruded at high shear rate, i.e. the host polymer.

The particle size of the nucleating agent in the case of such agentswhich do not dissolve in the molten polymer is preferably in the rangeof 0.5 to 20 micrometers. The smaller the particle size, the moreeffective is the nucleating agent in enabling the shear rate to beincreased at a given content of nucleating agent in the polymer.Examples of thermoplastic polymers which can be benefitted in accordancewith the present invention include the polyolefins such aspolypropylene, e.g. isotactic polypropylene, linear polyethylenes suchas high density polyethylenes (HDPE), linear low density polyethylenes(LLDPE), e.g. having a specific gravity of 0.89 to 0.92. The relativelynew linear low density polyethylenes made by the INSITE® catalysttechnology of Dow Chemical Company and the EXACT® polyethylenesavailable from Exxon Chemical Company can also be benefitted from thepresent invention; these resins are generically called (mLLDPE). Theselinear low density polyethylenes are copolymers of ethylene with smallproportions of higher alpha monoolefins, e.g. containing 4 to 8 carbonatoms, typically butene or octene. Any of these thermoplastic polymerscan be a single polymer or a blend of polymers. Thus, the EXACT®polyethylenes are often a blend of polyethylenes of different molecularweights.

Examples of fluoropolymers include the melt-fabricable copolymers oftetrafluoroethylene with one or more fluorinated monomers such asfluoroolefins containing 1 to 8 carbon atoms, such ashexafluoropropylene, and fluoro(vinyl ethers) containing three to tencarbon atoms, such as perfluoro(alkyl vinyl ether), wherein the alkylgroup contains 3 to 8 carbon atoms. Specific such monomers includeperfluoro(ethyl or propyl vinyl ether). Preferably the fluoropolymer isperfluorinated and has a melt viscosity of 0.5×10³ to 5×10⁶ Pa.s at 372°C. These fluoropolymers are perfluorinated, but less thanperfluorination can be used. For example, the fluorine content of thefluoropolymer is preferably at least 35 wt %. Examples of such polymerswhich are not perfluorinated and can be used includetetrafluoroethylene/ethylene and chlorotrifluoroethylene/ethylenecopolymers.

From the diversity of the thermoplastic polymers, ranging frompolyolefins to fluoropolymers, it is apparent that many otherthermoplastic polymers are useful in the present invention. All suchthermoplastic polymers have melt viscosities such that they aremelt-extrudible.

The extrusion process of the present invention produces an unfoamedextrudate and unfoamed articles such as wire insulation, tubing, filmand rods obtained from the extrudate. By extrusion of an unfoamedpolymer in the process of the present invention is meant that neitherthe extrudate nor its articles are foamed. The extrudate and articlesobtained from the extrudate may have a small percentage of voidsresulting from air or other gas entering the extruder with the polymerfeed, but such articles will nevertheless contain no more than 5% voidsand preferably less, e.g. less than 3% voids, which would not beconsidered as a foamed extrudate or foamed article.

The improvement in shear rate without roughening of the surface of theunfoamed extrudate is shown in the Examples.

EXAMPLES

The equipment for determining the shear rate at which smooth extrudatescan be produced involves both a rheometer and extrusion equipment. BothTeflon® FEP fluoropolymer resin and polyolefin resins were tested usingthese two sets of equipment.

The rheometers were standard laboratory rheometer equipment having a 90degree die inlet angle.

The extrusion equipment involved an 31.725 mm Entwistle extruder havinga 31/1 length to diameter ratio and equipped with a crosshead extrusiondie. Extruder screw was a design standard for Teflon FEP® fluoropolymerresins. The crosshead was a Nokia Maillefer 4/6 or a B & H 75. The NokiaMaillefer crosshead had included cone angles (14 and 28 in the FIGURE)for the die and tip ("tip" is the wire guide such as wire guide 16 inthe FIGURE) are equal at 60 degrees. The B & H crosshead was larger thanthe Nokia-Maillefer crosshead and had an included cone angle of the tipthat was about 14 degrees and a die inlet angle of about 17 degrees. Thedie orifice/die tip had the following diameters:

    ______________________________________                                                        Nokia Maillefer 4/6                                                                        B & H 75                                         ______________________________________                                        Die Internal Diameter                                                                         2.99 mm      3.10 mm                                          Tip Outer Diameter*                                                                           1.92 mm      1.92 mm                                          ______________________________________                                         *A portion of the tip corresponding to cylindrical extension 24 of the        figure.                                                                  

A Brabender extruder was also used for producing polyolefin film forrheology testing. Table I reviews this extrusion equipment, theEntwistle equipement and includes the temperature profiles used.

                  TABLE I                                                         ______________________________________                                        Extrusion Testing Conditions                                                  ______________________________________                                        Extruder Dia-          Entwistle                                                                              Brabender                                     meter (mm)                                                                    Diameter (mm)          37.73 mm 37.73 mm                                      L/D                    31/1     25/1                                          Screw Design           Std. FEP Barr screw                                              screw                                                               Crosshead/             Maillefer                                                                              Film Head*                                    Film Die               4/6                                                                           Crosshead                                                                              (6.34 mm                                                                      inlet opening)                                Die Details                                                                   Die                    2.99 mm  Film = 25.4 mm                                Tip                    1.52 mm  Wide & 0.508                                                                  mm thick                                      Temperatures                                                                            Teflon ® FEP                                                                           Polyolefin                                                                             Polyolefin                                              ° C.  ° C.                                                                            ° C.                                   Rear Barrel                                                                             371          163      150                                           Center Rear                                                                             371          163      180                                           Barrel                                                                        Center    371          163      200                                           Center Front                                                                            371          163      no zone on barrel                             Front     371          163      204                                           Adaptor   371          163      --                                            Head      371          163      204                                           Die       371          163      204                                           Melt      374-380      164-169  150-204                                       Screw Speed                                                                             5 to 100 RPM in       5 to 100 RPM in                                         5 to 10 RPM Steps     5 to 10 RPM Steps                             Melt Pressure                                                                           Varies with screw     Varies with screw                             at Crosshead                                                                            speed to              speed to 34.4 Pa                                        34.4 Pa                                                                       Maximum               Maximum                                       ______________________________________                                         *Die has the wellknown "coat hanger" configuration to provide a streamlin     flow of the molten polymer through the die and across its width (film         width).                                                                  

Comparative Example 1

Teflon® FEP fluoropolymer grades TE 3100 and TE 4100 as virgin resins aswell as TE 3100 containing 0.25% boron nitride and 0.011wt % calciumtetraborate (CTB) were tested in laboratory rheometers with the resultsshown in Table II and IIA. Two separate studies are listed, one at shearrates from 10 to 7000 reciprocal seconds (Table II) and the other from10 to 2000 reciprocal seconds. (Table IIA).

In summary, almost identical performance is noted with the virgin resinsand the boron nitride filled TE3100 resin in rheometer tests. Presenceof boron nitride and CTB show essentially no difference in the surfacecharacter of the unfoamed beading extrudate.

                  TABLE II                                                        ______________________________________                                        Rheometer Testing of Teflon ® FEP Fluoropolymer Resin 350° C.      Shear                                                                         Rate  Surface Character of Rheometer Beading                                  (Sec.sup.-1)                                                                        TE4100.sup.2 TE3100       TE3100                                        ______________________________________                                              Virgin       Virgin       0.25% BN                                            Resin        Resin        type SHP 325.sup.4                                                            110 ppm CTB                                   10    Smooth       Smooth       Smooth                                        30    Smooth       Smooth       Transitioning to                                                              Sharkskin                                                                     Sharkskin fracture                                                            related to die                                                                land starts here.                             100   Sharkskin    Sharkskin    Sharkskin                                     300   "            "            "                                             1000  Smooth.sup.3 Not reported "                                             2000  gross melt fracture                                                                        gross melt fracture                                                                        gross melt fracture                           4000  "            "            "                                             7000  "            "            "                                             ______________________________________                                         .sup.1 The shear rate through an annular orifice is calculated from the       gap distance between the die and tip surface (H) and the circumfrence of      the gap at its midpoint (πD, average diameter) from the following          equation: The numerator is 6q wherein q = volumetric flow rate of molten      FEP resin and the denominator is H.sup.2  × πD.                      .sup.2 Reference: Rosenbaum, Hatzikiriakos and Stewart "Flow Implications     in the Processing of Tetrafluoroethylene/hexafluoropropylene Copolymers",     Interpolymer Processing X, pub. by Hanser Publishers, Munich (1995).          .sup.3 Supershear per U.S. Pat. No. 2,991,508                                 .sup.4 Available from Carborundum Company                                

                  TABLE IIA                                                       ______________________________________                                        Teflon ® FEP TE-4100, TE-3100 fluoropolymer resins,                       each containing 0.025% BN type SHP325 and 110 ppm CTB                         Laboratory Rheometer Test at 350° C.                                                Smoothness of Extrudate                                          Shear Rate (sec.sup.-1)                                                                    TE-4100       TE-3100                                            ______________________________________                                        10           Smooth        Smooth                                             30           Smooth        Transition to rough                                100          Sharkskin     Sharkskin                                          300          Stick slip*   Sharkskin                                          1000         Semi smooth   Sharkskin                                          2000         Gross melt fracture                                                                         Gross melt fracture                                ______________________________________                                         *alternating smooth and gross melt fracture                              

Comparative Example 2

The TE3100 characterized above was evaluated in the Entwistle extruderusing the conditions listed in Table I with the Nokia-Maillefercrosshead. Instead of beading, the extrudate was in the form of anunfoamed extruded tube. No foam cell nucleating agent was present. Thesedata are listed in Table III.

Use of the Nokia-Maillefer crosshead provides a very streamlined flow ofmolten polymer. This postpones the point of sharkskin melt fracture fromless than 100 reciprocal seconds in rheometer beading tests toapproximately 300 reciprocal seconds in extrusion before melt fractureis encountered on the internal diameter surface of the tubing withvirgin (no nucleating agent) resin.

                  TABLE III                                                       ______________________________________                                        TE3100 (virgin) Melt Flow Rate 17.3                                           Maillefer 2.99 mm die and 1.52 mm Tip                                                                        Standard FEP Screw in Entwistle Extruder                    Melt                                                                                       Calculated                                                                                Smoothness                                                   Outputrature                                                                                      of Extrudate                         RPM                                           ID & OD                         ______________________________________                                            0.9                                                                                   371        5     100          Smooth ID & OD                      1.8                                             Smooth ID & OD                3.5                                             Smooth OD                                                                                    Sharkskin                                        ID                                          7.0                                             Smooth OD                                                                                Sharkskin ID       15.0             374                                                                                                         Sharkskin                      30.0             374                                                                                                         Sharkskin                      40.0             376                                                                                                         gross melt fracture            50.0             377                                                                                                         gross melt fracture            60.0             378                                                                                                         gross melt fracture            65.0             379                                                                                                         gross melt                     ______________________________________                                                                          fracture                                

In the following examples, it will be shown that completely differentperformance occurs in melt extrusion with foam cell nucleating agentspresent in the extruded polymer.

Example 1

Comparative Example 2 was repeated except that the TE3100 resincontained 0.25 wt % boron nitride and 0.011 wt % CTB. Both surfaces ofthe extruded tube were smooth (no sharkskin or gross melt fracture) fromthe low shear rate of 100 reciprocal seconds to >4500 reciprocalseconds. These data are in Table IV. The surface smoothness encompassesthe rheology range from sharkskin fracture well into the range of grossmelt fracture. The shear rate of this Example was at least 10× theearliest occurrence of surface roughness in Comparative Example 2.

                  TABLE IV                                                        ______________________________________                                                        TE3100, with 0.25% BN, Type SHP325, 110 ppm CTB               Maillefer 2.99 mm die, 1.52 Tip in Standard                                    FEP Screw in Entwistle Extruder                                                           Melt                                                                                       Calculated                                                                                Smoothness                                                   Outputrature                                                                                      of Extrudate                         RPM                                           ID & OD                         ______________________________________                                           0.9                                                                                   373         5        100                                                                                         Smooth                          1.8                                            Smooth                         3.5                                            Smooth                         7.0                                            Smooth                         15.0             374                                                                                                        Smooth                          30.0             374                                                                                                        Smooth                          40.0             375                                                                                                        Smooth                          50.0             376                                                                                                        Smooth                          60.0  377           263                       Smooth500                       ______________________________________                                    

In the subsequent Examples, the extrusion tests of resin samples (TableV data) were conducted in a similar fashion. In some of these examples,instead of presenting all the data from 5 to 70 screw RPM, the pointwhere the extrudate transitions from smooth to fractured is noted.

Example 2

Data in Table V show various foam cell nucleating agents allowing theunfoamed tubular extrudate smoothness to be extended to different levelsof shear rate before surface ski roughness appeared. The surfaceroughness appearing on the inner diameter (surface) of the extrudateusing 3300 ppm (0.33 wt %) BaS-10 was eliminated by increasing theBaS-10 concentration to 5000 ppm.

The additive providing the best extension of extrusion rate was 0.25%boron nitride and 330 ppm CTB. And specifically, the best boron nitrideis type PSSP150 and also type HCV. Examination at three magnificationsshow the surface of the extrudates containing the PSSP150 or the HCVboron nitride to be remarkably smooth and free of defects. So far, themaximum smooth surface performance has been to a 6000 reciprocal secondshear rate value with TE4100 containing BN type HCV. This is far beyondland fracture (sharkskin) and onset of die inlet fracture (gross meltfracture) noted in rheometer and extrusion testing of the virgin TE4100.

Some variability has been noted over four separate test extrusionevaluations of the same and also of different master batches of TE4100containing 400 ppm CTB. Similar variations were noted in extrusion testswith 400 ppm zinc borate in TE 4100. CTB seems to function with moreuniform performance when it is the secondary component as in BN/CTB.

                                      TABLE V                                     __________________________________________________________________________       Influence of Foam Cell Nucleating Agents on Rheology and Tubing            Surface Smoothness                                                            Extrusion Tests in the Entwistle Extruder using a Maillefer 4/6 Crosshead     and melt Extrusion                                                                                   Temperature of 383° C.  with no melt draw                            Maximum Shear Rate in sec.sup.-1 at which the                                 ID or OD Surface of the tubing is                                             Smooth*. In addition, the surface                                             smoothness is rated E to A. A rating                                          of A+++  denotes excellent surface                                            smoothness by the naked eye and by                                            scanning electron photographs at 25×,                                   500×  and 1000×.                                                                                        Outer                                   Diameter            Inner Diameter                       __________________________________________________________________________        TE4l00                                                                        3300 ppm BaS-10           3000B                                                                                         300A                            TE4100                                                                             5000 ppm BaS-10                                   >6500B                 TE4100                                                                                      400 ppm MgO                                                                                                           >6500B                  TE4100                                                                                     0.25%  talc +  110 ppm CTB                                                                    6500B                                                                                                   >6500B                 TE4100                                                                                      1.25% PTFE 110                                                                                                         2700B                  TE4100                                                                                      0.5% PTFE 1100                                                                                                         3000B                  TE4100                                                                                      0.5% PTFE 1100                                                                                                       900B                     TE4100                                                                                     800 ppm CTB                                                                                                            800B                    TE4100                                                                                      400 ppm CTB                                                                                                            >6500B                 TE4100                                                                                     400 ppm CTB                                                                                                              600B                  TE4100                                                                                      400 ppm CTB                                                                                            4000B-500B 4500-5000B*                 TE4100                                                                                      800 ppm BaS-10 and 100 ppm CTB                                                         4000B                           4000B                  TE4100                                                                                      2500 ppm BaS-10 and 100 ppm                                                                >6000B                     >6000B                  TE4100                                                                                      0.25% BN(3030) + 110 ppm CTB                                                                   >6000                                                                                                 >6000                  TE4100                                                                                     0.25%  BN (SHP325) 110 ppm CTB                                                           >5500B                        >5500B                  TE4100                                                                                     0.25%  BN (HCV) 110 ppm CTB                                                                >6000A+                     >6000A+                 TE4100                                                                                     0.25% BN (HCV) 330 ppm CTB                                                                   >6000A+++                                                                                             >6000A+++                 TE4100                                                                                     0.25% BN (PSSP150) 330 ppm CTB                                                          >3750A+++                    >3750A+++                 TE4100                                                                                     0.25% BN (PSSP151) 330 ppm CTB                                                            >3750A                       >3750A                  TE4100                                                                                     0.25% BN (PHPP325) 330 ppmk CTB                                                         >3750A                         >3750A                  TE4100                                                                                     0.03% BN (PSSP325) 30 ppm CTB                                                              >3750A                      >3750A                  TE4100                                                                                     0.03% BN (PSSPl51) 30 ppm CTB                                                             0 to 350A, 3500 to 5086A*                                                              0 to 350A, 3500 to 5086A                    __________________________________________________________________________          *Shear rate range denotes smooth surface extrudate production.               NOTE 1 The HCV is available from Advanced Ceramics Corp and the PSSP      150, 151, 325  and 3030 grades of BN are available from Carborundum           Company.                                                                          NOTE 2. The gap in surface smoothness shear rates, e.g. between 500       sec.sup.-1 and 4500 sec.sup.- 1   when 400 ppm CTB was used indicates tha     surface roughness was present at shear rates within the gap.                       NOTE 3. The E to A rating system for smooth surfaces is used in late     Examples.                                                                

Comparing Foam Nucleation Efficiency to Extrusion Rheology ImprovementPerformance

The efficiency of boron nitride as a foam nucleant and as a rheologyimprover for unfoamed extrudable is compared in Table VI.

                  TABLE VI                                                        ______________________________________                                                             Comparison of Foam Nucleation Vs. Rheology Improver                                         in Teflon FEP Fluoropolymer Resin                                 Rating as a                                                                             Rating as                                    Foam RG-62   (20 ft/min.)                                                                            Foam Cell a Rheology Im-                               core         % Voids   Nucleant  prover (unfoamed                             Av. Dia.     (and cells/                                                                             (from cell                                                                              Extrudate                                    Foam Cells   cc)       count)    Table V)                                     ______________________________________                                        0.5% BN,                                                                             12 mils   56%       Good    Good                                       type SHP         (32,000                                                      325              cells/cc)                                                    0.5% BN,                                                                             30 mils   59%       Poor    Excellent                                  type HCV         (25 cells/cc)     (6,000 A)                                  ______________________________________                                    

The Table Shows that the better nucleating agent for making a foamedextrudate (type SHP 325) is less effective in improving extrusionperformance of unfoamed extrudate, type HCV BN being more effective thantype SHP 325 in that regard. The HCV and PSSP150 types of BN werecharacterized by a very small particle size, less than 5 micrometers,usually 2-5 micrometers, while the SHP 325 type had a larger particlesize.

In addition, U.S. Pat. No. 4,764,538 shows the best foaming efficiencywith boron nitride (type SHP 325) occurs at 90 to 190 ppm added CTB. Thepresent rheology work shows the best performer is boron nitride (typePSSP150 and also HCV), but the CTB level should be near 330 ppm.

Example 3

Tests show the boron nitride can be reduced to levels of 0.025% (250ppm) and the good surface smoothness rheology performance is maintained.These data are shown in Table VII. In this test procedure, the entirespan of screw speed of the Entwistle extruder using the Nokia-Maillefercrosshead was studied as in the other examples. The data of the tabledenote points where the tube extrudate is smooth and where melt fracturereturned.

At very low values of boron nitride concentration, the shear ratemid-point of the zone where melt fracture re-occurs trends to lowervalues with lower concentrations of boron nitride. For example, at0.0125% BN the mid-point of the melt fracture zone is 3700 reciprocalseconds and the zone width is 500 reciprocal seconds. At 0.005%, thismid-point has drifted down to 1400 reciprocal seconds and the zone widthis 400 reciprocal seconds.

                                      TABLE VII                                   __________________________________________________________________________                          Teflon ®  TE 3100 FEP Fluoropolymer Resin           Influence of Boron Nitride/CTB Level upon Tubular Extrudate Surface                        Smoothness BN type SHP325-Melt Extrusion Temperature             383° C.                                                                                                                                  Maximum                          Shear Rate in sec.sup.-1                                                      to yield surface                                                               smoothness                                                                                       ID                      OD           __________________________________________________________________________                                 N                                                 Virgin                        300A                 500A                      TE3100                                                                        TE3100+                                                                                    0.25% (2,500 ppm) BN (SHP325) +                                                          >5500A                                                                                            >5500A                                                        110 ppm CTB                                       TE3100+                                                                                        0.125% (1200 ppm) BN (SHP325) +                                                     >5500A                                                                                             >5500A                                                  55 ppm CTB                                              TE3100+                                                                                        0.06% (600 ppm) BN (SHP325) +                                                         >5500A                                                                                           >5500A                                                     27 ppm CTB                                           TE3100+                                                                                0.025% (250 ppm) BN (SHP325) +                                                               >5500A                                                                                            >5500A                                                      11 ppm CTB                                          +    0.0125% (125 ppm) BN (SHP325) +                                                                0 to 3500A                                                                                      0 to 3500A                                                                4000 to 5500A                             +                0.0083% (83 ppm) BN (SHP325) +                                                      0 to 2500A                                                                                     0 to 2500A                                                                  2800 to >5500A                          +           0.0050% (50 ppm) BN (SHP325) +                                                            0 to 1200A                                                                                    0 to 1200A                                                                    1600 >5500A                           __________________________________________________________________________       *The gap between shear rates at which surface smoothness occurred was       characterized by surface roughness, e.g. in the shear rate range of 3500      to 4000 sec.sup.-1 for the composition containing 0.0125 wt % BN and 5 pp     CTB.                                                                     

Example 4

Teflon® FEP fluoropolymer grade 100, a resin of higher melt viscositythan TE 3100 and TE 4100, was checked for rheology performance in theEntwistle extruder using both the Nokia-Maillefer 4/6 crosshead and alsowith the B & H 75 crosshead. At comparable resin output, the meltpressure or shear stress in the Maillefer head was approximately 35%less.

The higher molecular weight of the FEP100 versus the resins described inthe previous examples makes the maximum shear rate limit for smoothsurface unfoamed tubing occur at a significantly lower value. Extrudatesurface smoothness could be obtained at higher shear rates by increasingthe melt temperature above 383° C.

The testing progressed to fully let down resin and then to added CTB,the point where melt fracture was at higher and higher shear ratesreaching a final level of 2900 reciprocal seconds. This probably occursdue to better and better mixing of the additive in the resin. These dataare shown in Table VIII.

                  TABLE VIII                                                      ______________________________________                                         Teflon ®  100 FEP Fluoropolymer Resin                                    Maximum Shear Rate (reciprocal seconds) for Smooth Surface                     Tube Extrusion Melt Extrusion Temperature 383° C.                                                                                        OD of                         Core    ID of Core                                         ______________________________________                                        Virgin FEP100                                                                                                     150A*               200A*                 FEP100+       0.5% BN (SHP325)                                                                                       1500A00A                               FEP100+       0.55% BN (SHP325)**                                                                                    1600A                                  FEP100+       0.25% BN (SHP325) +                                                                      2400A       2900A                                              110 ppm CTB***                                                      FEP100+       800 ppm BaS-10*                                                                                        2000B000B                              ______________________________________                                         *No supershear window in tests to 6000 sec.sup.-1  with virgin T100 resin     **Let down in extrusion testing at 9/l from a 5% BN concentrate, i.e. the     concentrate was melt blended in the Entwistle extruder as the extrusion       occurred.                                                                     ***Fully let down in a prior extrusion, i.e. the composition was blended      in a twinscrew extruder to form extruded cubes of the composition, and        this composition was fed to the Entwistle extruder.                      

Comparative Example 3

Entwistle extrusion tests on Teflon® TE3100 and TE4100 resin will resultin gross melt fracture point at the shear rates indicated using theNokia-Maillefer crosshead. The maximum wire coating speed for a finalFEP wall thickness of 0.007 inch (0.1777 mm) having smooth surfaced IDand OD on the melt cone are calculated from the meximum shear rate inthe table below.

    __________________________________________________________________________            Internal Diameter                                                                        Crosshead, Set #1                                                                        Crosshead, Set #2                               __________________________________________________________________________    Resin and MFR                                                                         Critical Shear Rate in                                                                   Maximum Extrusion                                                                        Test Extrusion Set up                           (Melt flow rate)                                                                      Maillefer crosshead                                                                      Rate       for 7 mil (0.177 mm)                                    Die = 0.118" (3.0 mm)                                                                    for 7 mil (0.177 mm)                                                                     wall on AWG 24 solid                                    Tip = 0.60" (1.5 mm)                                                                     wall on AWG 24 solid                                                                     wire (0.0201", 0.51 mm)                                            wire (0.0201", 0.51 mm)                                            371° C. 700° F.                                                            Die = 0.238" (6.0 mm)                                                                    0.145" (3.7 mm)                                         (reciprocal seconds)                                                                     Tip = 0.135" (3.4 mm)                                                                    0.078 (2.0 mm)                                                     Draw down ratio 51/1                                                                     Draw down ratio 20/1                                               Draw balance = 1.04                                                                      Draw balance = 1.10                             TE3100 18 MFR      343 ft/min (104 m/min)                                                                   87 ft/min (27 m/min)                            TE4100 22 MFR      610 ft/min (186 m/min)                                                                   154 ft/min (47 m/min)                           __________________________________________________________________________

Example 5

Comparative Example 3 was repeated, except that the TE3100 was meltcompounded with a concentrate made in a twin screw extruder. Theconcentrate contained 0.25% boron nitride type SHP 325 and 110 ppm CTB.It was let down with additional TE3100 in the Entwistle extruder and putto cubes having a final loading of 0.06% BN and 27 ppm CTB and having amelt flow rate of 18.

This resin was extruded as unfoamed insulation onto wire using a 0.145inch (3.68 mm) die and a 0.078 inch (1.98 mm) tip using the NokiaMaillefer cross-head on a 45 mm Nokia Maillefer extruder. The extruderscrew has a standard design. The extrusion condition on the 45 mm lineis as follows:

    __________________________________________________________________________         Center    Center                                                         Rear     Rear                                                                              Center                                                                           Front                                                                               Front                                                                              Clamp                                                                             Adaptor                                                                           Crosshead                                  __________________________________________________________________________         602° F.                                                                 700° F.                                                                    700° F.                                                                      700° F.                                                                     700° F.                                                                     685° F.                                                                     684° F.                                                                     755° F.                            (316° C.)                                                                    (371° C.)                                                                  (371° C.)                                                                   (371° C.)                                                                   (371° C.)                                                                   (363° C.)                                                                   (362° C.)                                                                   (401° C.)                           __________________________________________________________________________

The screw speed was 20 rpm, and the melt temperature was 708° F. (376°C.), and a melt pressure of 9.4 MPa.

The extrusion was successfully run to smooth inner and outer insulationsurfaces at 851 ft/min (258 meters per min). This corresponds to a shearrate of 2200 reciprocal seconds. This shear rate and wire speed wereabout 10 times greater than what could be achieved with this samedie/tip set up using virgin TE3100 FEP resin and smooth surfaced wireinsulation. The electrical flaw count associated with the coated wireamounted to only one flaw per 3000 ft (909 meters).

Example 6

Comparative Example 3 was repeated, except that the TE4100 was meltcompounded from a concentrate made on a twin screw extruder. Theconcentrate contained 0.25% BN, type HCV. It was let down withadditional TE4100 in the Entwistle extruder to a final pelletedcomposition containing 0.05% BN, and was of melt flow rate 23.

In this case, the BN modified TE4100 resin could be extruded at 5000reciprocal seconds and 2000 ft/min (609 meters/min).

The extrusion rate of 2000 ft/min (609 m/min) was about thirteen times154 ft/min (47 m/mm) rate associated with smooth surfaced (ID & OD) wireinsulation of the virgin TE4100 in tooling #2 of the Table. The boronnitride agent allowed this higher speed extrusion at low draw down(24/1) in Comparative Example 3 and high shear rate (4300 sec⁻¹).

The extrusion conditions on the 45 mm Nokia-Maillefer extrusion linewere as follows:

    __________________________________________________________________________         Center    Center                                                         Rear     Rear                                                                              Center                                                                           Front                                                                               Front                                                                              Clamp                                                                             Adaptor                                                                           Crosshead                                  __________________________________________________________________________         690° F.                                                                 700° F.                                                                    700° F.                                                                      700° F.                                                                     700° F.                                                                     730° F.                                                                     730° F.                                                                     750° F.                            (366° C.)                                                                    (371° C.)                                                                  (371° C.)                                                                   (371° C.)                                                                   (371° C.)                                                                   (388° C.)                                                                   (388° C.)                                                                   (399° C.)                           __________________________________________________________________________

Screw Rpm was 42, the melt 757° F. (402° C.) with a melt pressure of 5.5MPa through the die of 0.145" (3.7 mm) and tip 0.075" (1.9 mm).

The wire insulations of Comparative Example 3 and Examples 5 and 6 wereunfoamed.

Comparative Example 4

LLDPE polyethylene resin DFDA7047, a standard in rheology studies, wasused along with a metallocene catalyzed polyethylene grade, coded Exact®3028, for the base virgin resins studies in the Entwistle extruder. TheNokia-Maillefer crosshead used in the extrusion studies had enlarged to3.10 mm die opening while the 1.92 mm tip was still the same size tomake unfoamed tubular extrudate.

In laboratory rheometer studies, each of these resins possess a criticalshear rate under 100 reciprocal seconds. Using the Entwistle extruder,the following results were obtained:

    ______________________________________                                                       Shear Rate                                                                    (Reciprocal seconds)                                                          DFDA7047  Exact ® 3028                                     ______________________________________                                        Smooth Tubing Extrudate                                                                        70          44                                               Zone of Sharkskin Melt Fracture                                                                114 to 930  90 to 144                                        Zone of Gross Melt Fracture                                                                    1812 to 3450                                                                              280 to 2400                                      ______________________________________                                    

Examples 6 and 7

High boron nitride compositions of 1000 ppm CTB and 2, 5 and 10% boronnitride, type SHP325, all allowed the unfoamed tubing of DFDA7047polyethylene to be smooth to approximately 1000 reciprocal seconds whichis much greater than the 114 sec⁻¹ for this resin as shown above. Thesecompositions were run under the conditions of Table I for polyolefinresins.

The following compositions of Exact® 3028 resin were also found to formsmooth-surface unfoamed tubing at 1000 reciprocal seconds:

1% boron nitride, type SHP325, and 500 ppm CTB

0.5% boron nitride, type SHP325, and 250 ppm CTB

Following this extrusion, a composition containing the Exact® resinwhich also contained 0.05% boron nitride, type SHP325 and 10 ppm CTB wassuccessfully run as unfoamed tubing for a two hour period at 627reciprocal seconds and was completely free of any melt fracture, ascompared to only 90 sec-1 for this resin containing no nucleating agent.

Following this, virgin Exact® resin 3028 was run as unfoamed tubing atthe same shear rate for an additional two hour period completely free ofmelt fracture. The extruder hold up was about 200 grams of polyethylene.At this shear rate, the output in grams per minute was 21 grams perminute or 2520 grams in two hours. The held up resin in the extruderchanged over about a dozen times during the two hour period. During thisrun with virgin 3028 resin, the amount of BN and CTB present in theextruder from the preceding run was continuously reduced to much smalleramounts, e.g. 0.005 diminishing to 0.001 wt % for the BN, and theextrudability improving effect of the BN was still exhibited by theextrudate. Photographs at 1000× showed boron nitride particles in theouter surface of the tubing after this two hours of running the virgin3028 resin.

Example 8

LLDPE polyethylene GRSN7047 resin powder was evaluated with and withoutthe boron nitride additive in the film extrusion equipment described inTable I. The gross melt fracture occurring a 1240 reciprocal secondswith the virgin polyethylene resin disappeared with 0.2% (2000 ppm)boron nitride, type SHP325 and 100 ppm CTB. Sharkskin melt fractureoccurred to the limit of testing, 2800 reciprocal seconds for thepolyethylene resin containing the nucleating agent, and gross meltfracture never occurred. Data are in Table IX.

                  TABLE IX                                                        ______________________________________                                                       Polyethylene Type GRSN7047 Film Extrusion                                          without and with boron nitride additive                                               GRSN7047                                          Rheology                                          0.2% BN                     Shear Rate                                    Type SHP325                     Reciprocal Seconds                                                                             Virgin GRSN7047                                                                                100 ppm CTB                                 ______________________________________                                                   200                                                                                        Smooth                                                                                  Not run                                     440                                    Sharkskin                              1040                        Sharkskin                                                                               Sharkskin                               1380                                  Sharkskin                               1600                      Gross Melt Fracture                                                                 Sharkskin                                     2200                      Gross Melt Fracture                                                                 Sharkskin                                     2600                      Gross Melt Fracture                                                                Sharkskin                                      ______________________________________                                    

In many cases of production, the sharkskin surface with it's sandpapertexture can be tolerated and in some rare cases, preferred. The grossmelt fracture, on the other hand cannot be tolerated because the surfacewould have unacceptable surface smoothness. With gross melt fracturethere would be an unacceptable surging of molten resin in filmproduction leading to very undesirable thickness variations. Theseobservations apply to all thermoplastic polymers.

Example 9

In this Example the unfoamed extrudate is in the form of beading (solidextrudate) and the extrusion equipment is a rheometer having a die inletangle of 90 degrees and die orifice 0.762 mm in diameter and the dieland length-to-diameter ratio was 40:1. In one experiment, the polymerused was TE4100 fluoropolymer, which could be extruded at a shear rateup to 55 sec⁻¹ before the beading surface exhibited a sharkskinappearance. The extrusion melt temperature was 325° C. This experimentwas repeated except that the TE4100 was premelt blended with foam cellnucleating agent: 0.25 wt % BN, 0.011 wt % CTB, and 0.008 wt % BaS-10.Surface roughness did not occur until the shear rate reached 70 sec⁻¹.As shear rate increased about 70 sec⁻¹, the surface roughness(stick-slip) was at a much lower degree than the surface roughnesspresent as the shear rate increased above 70 sec⁻¹ for the fluoropolymercontaining no foam cell nucleating agent. Gross melt fracture occurredat 1350 sec⁻¹ for the resin without foam cell nucleating agent, whilegross melt fracture was delayed until 1500 sec-1 when the resincontaining the foam cell nucleating agent was present.

Whereas previous rheometer extrusions did not show improvement when foamcell nucleating agent was present, the improvement obtained in thisseries of experiments indicate the effectiveness of particular agents toproduce laminar flow where the equipment might otherwise produce onlyturbulent flow.

We claim:
 1. Composition comprising thermoplastic polymer and 0.001 to0.015 wt % boron nitride, said thermoplastic polymer being polypropyleneor linear polyethylene.
 2. Composition of claim 1 wherein saidthermoplastic polymer is linear polyethylene.
 3. Composition comprisingfluoropolymer and 0.001 to 0.015 wt % of boron nitride.
 4. Compositionof claim 3 wherein said fluoropolymer is perfluoropolymer. 5.Composition of claim 1 having thermally stable inorganic salt presenttherein.
 6. Composition of claim 5 wherein the concentration of saidsalt is 0.002 to 0.04 wt %.
 7. The composition of the claim 1 whereinthe amount of said boron nitride is 0.005 to 0.01 wt %.